Why doesn't Blender detect RTX 2060 with OpenCL?
That is because Cycles in Blender 2.8x only uses OpenCL for AMD GPUs. For Nvidia GPUs you have to select either CUDA or OptiX (only for RTX cards until 2.90, then for Maxwell GPUs and higher). This is documented in Blender's manual.

Nvidia
CUDA and OptiX are supported for GPU rendering with Nvidia graphics cards.



AMD
OpenCL is supported for GPU rendering with AMD graphics cards.